Firstly I would say that there is somewhat a dislike of link-only answers as in "go there, try that". Posting them as comments avoids the implied frowning that the auto-commenter that people (myself included) use from adding these words as a comment.
Whilst this may theoretically answer the question, it would be preferable to include the essential parts of the answer here, and provide the link for reference.
Effectively a link-only answer is lazy, and a comment is a lazier way of asking "Does this fix it? If so I might post a real answer."
It doesn't risk a downvote if the link is bad, or otherwise wrong. It also absolves you to some degree of actually putting some effort in to actually answering the question.
I dislike link-only comments, but hate link-only answers more because you could have tried to pull in the useful information, but didn't.
The auto-commenter does put a bit of stigma on link only answers and, if it pushes someone to comment rather than actually answer the question, effectively makes way for someone to answer the question properly. That in my mind is a Good Thing™.
If on the other hand it is an actual proper helpful answer I would prod the person involved to post it as an actual answer so that it can receive the upvotes it rightly deserves. In that case I cannot really understand why they would post an answer as a comment as it is just as easy to do either.
Whether it is a link or an answer in comment form, if it is truly useful it should be a proper answer and more than just a "go there" as you are detracting from the usefulness of this site as a knowledgebase. People come here for answers and should get more than "why don't you try somewhere else" as an answer.
My preference would be to make all link-only answers into comments, but that's just me.